Fix T70615: Cycles ignores BSDF inputs when nodes are optimized

When compiling BSDF nodes, we only assing stack space to the normal and
tangent inputs if they are linked. However, it could be that the
ConstantFolder removed the link, so checking if there is a link fails
to take this into account.

To fix this, added a flag to ShaderInput to keep track of whether a
constant was folded into the input, and use it as well to verify that
the socket is linked when assigning stack space.

Reviewed By: brecht

Maniphest Tasks: T70615

Differential Revision: https://developer.blender.org/D11731

intern/cycles/render/constant_fold.cpp

@@ -48,6 +48,7 @@ void ConstantFolder::make_constant(float value) const
 
   foreach (ShaderInput *sock, output->links) {
     sock->set(value);
+    sock->constant_folded_in = true;
   }
 
   graph->disconnect(output);
@@ -59,6 +60,7 @@ void ConstantFolder::make_constant(float3 value) const
 
   foreach (ShaderInput *sock, output->links) {
     sock->set(value);
+    sock->constant_folded_in = true;
   }
 
   graph->disconnect(output);

intern/cycles/render/graph.h

@@ -79,7 +79,11 @@ enum ShaderNodeSpecialType {
 class ShaderInput {
  public:
   ShaderInput(const SocketType &socket_type_, ShaderNode *parent_)
-      : socket_type(socket_type_), parent(parent_), link(NULL), stack_offset(SVM_STACK_INVALID)
+      : socket_type(socket_type_),
+        parent(parent_),
+        link(NULL),
+        stack_offset(SVM_STACK_INVALID),
+        constant_folded_in(false)
   {
   }
 
@@ -111,6 +115,10 @@ class ShaderInput {
   ShaderNode *parent;
   ShaderOutput *link;
   int stack_offset; /* for SVM compiler */
+
+  /* Keeps track of whether a constant was folded in this socket, to avoid over-optimizing when the
+   * link is null. */
+  bool constant_folded_in;
 };
 
 /* Output

intern/cycles/render/svm.cpp

@@ -304,7 +304,7 @@ int SVMCompiler::stack_assign(ShaderOutput *output)
 
 int SVMCompiler::stack_assign_if_linked(ShaderInput *input)
 {
-  if (input->link)
+  if (input->link || input->constant_folded_in)
     return stack_assign(input);
 
   return SVM_STACK_INVALID;